The reason I asked for the seat seam is, I think the back waistline is v-shaped because it´s sewn in a wrong angle and not necessarily because of a too short seat seam. For me it looks like you took out about 1inch on each side of the seatseam wich is too much when the waistline and side seam are kept the same.
A waist-hip drop of 9inches is indeed a lot. A proportional pattern is meant for a drop of about 5 inches. My approach would have been to draw a pattern with a propotional waist and than look where to get rid of the additional drop. Considering your figure I would install a second back dart (wich you did) but would also remove something at the sides seams so the seamlines would be a bit more curvy between hip line and waist line. (Drawing the pattern with a proportional waist instead of the measured one will probably have an inpact on the positioning of the fly seam and belly point)
BTW: I don´t think you have a flat butt. It´s at least normal. So a greater seat angle would be a good idea.
One last note: remeasuring your underside draft I noticed your crease line in the pattern is by far not the center line of your pattern. When I draw a line through the outseam crotch line notch wich is perpendicular to the crease line, the outseam is at 6,5cm and the crotch point is at 8,7cm. Does your pattern instructions call for this?